The present disclosure relates to a method for slewing a body from an initial state to merge with a desired state profile. In particular, the present disclosure relates to slewing a spacecraft.
It is often necessary to reorient, or slew, a moving body from a first state to merge with a time profile of desired future states, where a state is defined as a combination of the body's attitude (3 dimensional angular orientation), and the rates of change of the attitude (3 dimensional angular velocities). For example, a spacecraft or some other body in space, air, water or on land may need to be reoriented so as to track one or more objects, such as missiles, that are moving relative to the body, where tracking is defined as pointing a body vector (such as the boresight of a sensing instrument) at the object. Also, when tracking is finished, the spacecraft may need to be reoriented to a different state determined by any of a variety of operational goals. It is important to note that the desired state depends upon the time when the state is achieved, hence the reference to a time profile of desired states, rather than just a single desired state.
In many situations, it is desirable to slew the spacecraft in a manner that is optimized both from the standpoint of the amount of time that it takes as well as from the standpoint of energy usage. For example, it can be desirable to perform the slew in a manner that minimizes both time and energy usage. Furthermore, it is desirable to formulate a slew that may be performed smoothly and stably by the spacecraft. Such a slew must conform to certain angular rate and acceleration limits.
To perform a slew, a spacecraft processor generates attitude commands, which are fed into an attitude control system (ACS). An attitude determination system (ADS) uses sensors and software methods to estimate the spacecraft's current actual attitude. The ACS uses actuators, for example reaction wheels, along with software methods to provide torques that keep the estimated attitude as close as possible to the commanded attitude. As mentioned, the ability to optimize time and energy usage during the slew is highly desirable, as is the ability to provide attitude commands that the ACS can follow without lag, oscillation, or instability. Improved control of this process can provide an increase in maneuver performance and accuracy, as well as energy efficiency.